Thistles of Cook County By Ellen Sobieck (US Forest Service) Cook County has four non-native invasive thistles: three biennials (Plumeless, Musk, and Bull thistle) and one perennial (Canada thistle). The two most common thistles (Bull and Canada) have become established in many areas of the county where they crowd out native plants and limit human recreation. Due to their aggressive character, these non-native thistles have been the target of control efforts by local residents and agencies. Many individuals participating in these invasive thistle management activities may not realize we also have native thistles in the Arrowhead Region. Three native thistle species that have been reported in Cook County: Cirsium muticum (Swamp thistle), Cirsium flodmanii (Flodman thistle), and Cirsium pumilum var. hillii (Hill’s thistle). The most abundant native thistle in Cook County is the Swamp thistle. As it names suggests, it is found in damp low-lying woodland areas, wet meadows, and near rivers. It is a long-stemmed plant noted for its ribbed stem and widely Swamp Thistle spaced, nearly spineless leaves. These features distinguish it from other native and non-native thistles. Flodman thistle is more commonly found in western Minnesota but can www.uwgb.edu be found sporadically in the Arrowhead region (See Jack Greenlee’s article). It prefers damp, www.minnesotawildflowers.info open areas, such as meadows, prairies, and pastures. Similar to Swamp thistle, Flodman thistle’s spines are less dense and prickly than non-natives thistles. Flodman thistle stems have a mottled white appearance due to its long, woolly, www.minnesotawildflowers.info www.keiriosity.com Continued on page 4 white hairs becoming tangled into bunches. Its leaves are shiny green Flodman Thistle on top and white-woolly on the underside. Its leaves are almost at right angles to the stem resulting in its distinctive "flipping appearance”. Compared to the other native thistles, Flodman thistle is the most prone to becoming weedy and has been declared a noxious weed in at least one Canadian province. Hill’s thistle is Cook County’s spiniest and shortest native thistle reaching only two feet at maturity. In addition, its leaves are oblong to elliptical with serrated edges and www.minnesotawildflowers.info www.geoclio.org tiny dense spines along the edge. Hills’ thistle prefers open, dry, sandy areas where it plays a key role in stabilizing soil after disturbance. Unfortunately, Hill’s thistle has been replaced by more aggressive non-native species in www.minnesotawildflowers.info many of these areas leading to its placement on Hill’s Thistle Minnesota’s Species of Special Concern list in 1984. Native and non-native thistles often occupy the same habitat in Cook County, which can make it difficult to www.minnesotawildflowers.info differentiate between www.keiriosity.com them. Please review the following and share it with others participating in invasive plant management to help recognize and identify native thistle populations in the field. www.keiriosity.com www.minnesotawildflowers.info Continued on page 5 Flodman thistle: Another thistle to watch for in Cook County Apparently thistles are a timely topic. In the Unlike the other thistles commonly found in same issue I was working with Ellen Sobieck on northeast Minnesota, the undersides of Flodman her thistle identification article, I received this thistle leaves are covered with a thick layer of information from Jack Greenlee. grayish-white hairs. It can be distinguished from Canada thistle (Cirsium arvense - non-native) and By Jack Greenlee (US Forest Service) swamp thistle (Cirsium muticum – native) Thistles are tough to identify, so when my weed because the flowering head on these two species crew excitedly told me they found a thistle species does not have any spines, while the flowering they had not seen before, I was skeptical. It was head of Flodman thistle does have spines. Three growing in a gravelly open area next to some busy other non-native thistles (Cirsium vulgare – bull railroad tracks near Idington, about halfway thistle, Carduus acanthoides – plumeless thistle, between Virginia and Cook, MN, and they Carduus nutans – musk thistle) in northeast collected some. After looking at the plants they Minnesota also have spines on the bracts that brought back to me, I had to agree: this thistle is surround the each flower head, but the spines on different. Flodman thistle are the smallest of the bunch. It turned out to be Flodman thistle (Cirsium According to the Minnesota Department of flodmanii), a thistle native to North America and Natural Resources vascular plant checklist found mainly in the northern Great Plains. The (MNTaxa, available online at: habitat for this species is prairies, meadows, and http://www.dnr.state.mn.us/eco/mcbs/plant_lists.h pastures, often found in damp soil. The sources I tml), Flodman thistle is actually known to occur in checked said it is more competitive than most southwest Cook County. If you are interested in other native thistles, so it has potential to infest learning more about thistle identification, check large areas and behave more like an invasive out this link – The Thistles of North Dakota species. (http://www.ag.ndsu.edu/pubs/plantsci/weeds/w11 20/w1120w.htm). This publication has all the Identification: Flodman thistle is a perennial species that occur in northeast Minnesota and is a thistle 2-3 feet tall with a creeping root system. great identification tool. Northeastern Minnesota thistles with spiny heads (left to right): Flodman, Bull, Plumeless, Musk Distribution of Flodman thistle NE MN thistles without spiny heads (left to right): Canada, Swamp Identification Information Habitat Deducing thistles Cirsium • Biennial; blooms July- September • Found in wetter habitat and has • greater space between leaves than muticum Stem: 3-5 ft. tall, green, ribbed with fine hairs Swamps, low- • other native thistles Leaves: alternate, deeply lobed, nearly lying meadows spineless, often widely spaced on the stem • Lacks bract spines and woolly- (Swamp thistle) and • Lower leaves up to 8” long by 3” wide white stem of Flodman’s. woodlands • Flowers: dark purple to lavender, up to 1.5” long • Taller and fewer spines on its Native with dark green almost spineless bracts leaves than Hill thistle • Biennial or short-lived perennial; blooms June- • Like most natives, its spines are Cirsium August less dense/prickly than the non- flodmanii • Stem: 2-3 ft. tall, slender with fine velvety natives coating • Its leaves are at nearly right angles (Flodman’s • Leaves: alternate, gray-green, deeply lobed with Sun; moist to stem, which creates its distinct thistle) white-woolly undersides and short spines at prairies “flipping appearance”, and edges. Lobes generally triangular and base differentiates it from other native leaves up to 6” long and non-native thistles Native • Flowers: pinkish to deep purple, up to 1-1.5” across with slender spines from bracts • Perennial; blooms June-August • Short in stature and singular flower Cirsium • Stem: 1- 2 ft. tall, stout, woolly, often single or head contrasts with other natives pumilum var. slightly branched Sun; open • The spiniest of native thistles; lacks hillii • Leaves: alternate, lobed with wavy edges, fields and the spiny stem wings of the Bull oblong to elliptic, with dense white downy prairies, sandy thistle and the sparse pubescent, (Hill’s thistle) undersides. Edges have many short spines. soil, disturbed branching stem of the Canada Leaves up to 8” in length areas thistle. Native • Flowers: reddish to deep purple singular flower at top of stem ranging 1.5-3” across • Perennial; blooms June-September • Young plants do NOT develop as • Stem: 2-5 ft. tall, light green, slender, spiny, rosettes Cirsium arvense sparsely pubescent, branches at top • Abundant flower heads • Leaves: alternate, up to 6” long with spines • Forms large colonies from (Canada thistle) along its wavy edges. Lower leaves deeply Sun; disturbed extensive horizontal root system lobed with spine at each tip while upper leaves areas, old • Differ from natives by lacking white narrower and toothed. Sparse hair on leaf fields striped bracts on its flower heads Non-native undersides • Lacks woolly pubescence Invasive • Flowers: lavender to pink, occasionally white surrounding the bracts which is Flower heads up to 3/4” across with purplish characteristic of the native Swamp bract tips. Noted for numerous heads per plant thistle • Biennial; blooms June-September • Lacks the native thistles’ white- Cirsium vulgare • Stem: 2-6 ft. tall, hairy, covered w/ small spine- striped bracts on their flower heads tipped leaves called “stem wings” • The bull thistle’s spiny stem wings, (Bull Thistle) • Leaves: alternate, up to 6” long, lobed w/coarse Sun; dry fields, hairy upper side and woolly hair on upper surface and woolly underside. disturbed underside separate it from other Long, sharp spines at midrib and lobe tips areas invasive thistles such as Canada Non-native Flowers: reddish to purple 1 ½-2” across. thistle Invasive Spines extend around base; bracts also tipped with spines .